Stimulation of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception.

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.